Cardio mapping system and method for cardio mapping

ABSTRACT

A method and system for determining the mechanism of cardiac arrhythmia in a patient is disclosed. The method basically entails measuring the impedance of cardiac tissue in a portion of the patient&#39;s heart using a catheter during an episode of supraventricular tachycardia to produce an iso-impedance map of that cardiac tissue on a video display and analyzing the pattern of the iso-impedance map to differentiate focal arrhythmia caused by a circumscribed region of focal firing and reentrant arrhythmia caused by a macroreentrant circuit. The method can also be used to identify regions of coherent rapidly conducting tissue e.g., Bachman&#39;s bundle or the inferoposterior pathway insertion points, to identify focal “mother rotors” throughout the left atrium that may participate in the generation and maintenance of atrial fibrillation and to identify areas of CAFE (complex atrial/fractionated electrograms) that truly reflect these mother rotors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility application is a continuation under 35 U.S.C. §120 ofnon-provisional patent application Ser. No. 13/604,168 filed on Sep. 5,2012, and entitled Cardio Mapping System and Method for Cardio Mapping,now U.S. Pat. No. 8,644,917, which in turn claims the benefit under 35U.S.C. §119(e) of Provisional Application Ser. No. 61/536,730 filed onSep. 20, 2011 entitled Cardio Mapping Method, the entire disclosures ofwhich are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

“Not Applicable”

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK

“Not Applicable”

FIELD OF THE INVENTION

This invention relates generally to electro-anatomic mapping of theheart and more particularly to systems and methods for impedance mappingof endocardial and epicardial surfaces of the heart to identify themechanism and site of origin of various arrhythmias, and to identifydamaged cardiac tissues (scar), as well as coherent, rapidly conductingtissue.

BACKGROUND OF THE INVENTION

Electrical contact mapping of the heart typically involves voltage andactivation mapping and is accomplished using a standard multi-polarelectrode catheter, e.g., a Biosense Webster deflectable tipmapping/ablation catheter (2 mm or 4 mm tip). The catheter is coupled toa processing unit or analyzer, which in turn is coupled to a videodisplay unit. In use, the catheter is inserted, via the Seldingertechnique, in the femoral veins and is positioned under fluoroscopicguidance at predetermined locations in the right atrium, rightventricle, coronary sinus and, if necessary, in the left atrium and leftventricle. The mapping catheter is then translocated to between 50 and200 different point locations throughout the cardiac chamber of interestduring the spontaneously occurring or induced arrhythmia which is eithera supraventricular tachycardia (SVT), a ventricular tachycardia (VT) orfrequent ventricular premature complexes (VPC's). At each point, withthe catheter tip in good contact with the endocardial wall of thechamber of interest, the following electrical parameters are measuredand registered by the software in the processing unit to their positionin 3-dimensional space on the endocardial surface of the chamber ofinterest: (1) local electrical activation time (LAT) and (2) tissuevoltage (V). That system is also capable of measuring tissue impedance(Z). The measurement of tissue impedance, being for the purpose ofdifferentiating scar tissue from normal tissue or fat from scar tissue,and these measurements are made during normal sinus rhythm.

An electro-anatomic activation map (such as shown in FIG. 2) isgenerated for the LAT and tissue voltage and those parameters aredisplayed on the video display as an iso-activation and an iso-voltagemap. The iso-activation map is evaluated for the pattern of activationas either being centripetal or reentrant. A centripetal pattern is onehaving a focal area of earliest activation with waves of progressivelylater activation spreading out concentrically from the earliestactivation site. This is compatible with a focus of electrical activityfiring off rapidly and activating the rest of the chamber sequentially.The other pattern of activation, i.e., reentrant, shows a well definedregion where the tissue with the earliest activation time is immediatelyadjacent to tissue with the latest activation time, indicating that thechamber is being activated sequentially, and continuously as a largereentrant circuit. The iso-activation map demonstrates activationemanating from one region and then sequentially spreading throughout thechamber and finally the returning to the region of earliest activation,as if inscribing a large circle of the spreading electrical wave front.

Heretofore iso-activation maps have typically been color coded so thatred indicates early activation sites, blue and purple indicates lateactivated sites, and orange, yellow and green indicates intermediateactivation sites.

Relying on these different patterns of the iso-activation map todifferentiate these arrhythmia mechanisms so that appropriate therapy,e.g., ablation, can be applied to the patient can sometimes bemisleading and can cause a lot of wasted time and energy in the effortto define the mechanism of the arrhythmia and direct the ablativetherapy. The major problem is that a focal arrhythmia can mimic areentrant arrhythmia, particularly when the focal firing tissue is inanother chamber and the electrical wave fronts that travel into thechamber being mapped cause activation in the chamber of interest in amacroreentrant pattern due to anatomic/physiologic barriers thatgenerate one-way conduction in that chamber. Thus, the iso-activationmap in the chamber of interest shows the macroreentrant pattern ofearliest activated tissue adjacent to latest activated tissue, while theactual arrhythmia generator is a group of cells focally firingelsewhere. Ablating across the presumed reentrant circuit in thisscenario to produce a “line of block” to interrupt the reentrant circuitisthmus and terminate the arrhythmia, will have no effect.

There is reason to believe that the above scenario, particularly if itinvolves the right atrium, is not so infrequent, as in this chamberthere are natural anatomic barriers that can confine the conduction ofelectric current to a fixed pathway, that would mimic a reentrantactivation pattern, with a little help from some physiologic barriersthat develop when there is associated organic heart disease that causefibrosis which can lead to anisotropic conduction and physiologic block.

Thus, there presently exists a need for additional methods and systemsfor identifying/differentiating arrhythmia sources, e.g., discriminatingbetween focal arrhythmias and reentrant arrhythmias. Additionally, thereis utility in identifying coherent, rapidly conducting pathways that maybe participating in reentrant circuits and to identify damaged cardiactissues, i.e., scar tissue, that is often the substrate formicro-reentrant circuits. The subject invention addresses those needs.

SUMMARY OF THE INVENTION

One aspect of this invention is a cardiac arrhythmia discriminationsystem for determining the mechanism of cardiac arrhythmia in a patient.The cardiac arrhythmia discrimination system comprises a catheter and anassociated processing unit. The catheter and the processing unit arearranged to measure the impedance of cardiac tissue of the patient atvarious selected points on the endocardial/epicardial surface of thepatient's heart during a spontaneously occurring or induced arrhythmia,e.g., supraventricular tachycardia, and providing the geometric positionof each of the points on the patient's heart. Each of the impedancemeasurements and point positions are recorded by the system. The systemis arranged to determine if a point on the patient's heart exhibits lowimpedance (Z_(low)), wherein Z_(low)≦Z_(min)+0.1(Z_(max)−Z_(min)), whereZ_(min) is the minimum impedance measured and Z_(max) is the maximumimpedance measured.

The system is also arranged to discard from its impedance measurementsany points associated with tissue voltage of less than 0.5 mV indicatingpoor contact of the catheter to the cardiac tissue, as well as anypoints that are too internal to the anatomic shell (e.g., >1 mm internalto a 15 degree spherical arc of curvature with a radius of 1.5-2 cminscribed by a local group of measured points on the heart chambershell) or points with far-field low amplitude electrograms, andevaluates the remaining points of measured impedance to determine ifthere is an area of approximately 3.4±2 cm² or 2.4±1.8% of the atrialsurface area having plural Z_(low) points therein.

In accordance with another aspect of this invention the system includesa video display coupled to the processing unit and which is arranged toproduce an iso-impedance map of the cardiac tissue. The iso-impedancemap is colored to represent differing impedances measured by thecatheter to enable a user of the system to visually analyze the colorpattern of the iso-impedance map to differentiate a focal arrhythmiacaused by a group of cells focally firing, from a reentrant arrhythmiacaused by a macroreentrant circuit.

In accordance with another aspect of this invention there is provided amethod determining the mechanism of cardiac arrhythmia in a patient. Themethod entails measuring impedance of cardiac tissue of the patient atvarious selected points on the patient's heart using a catheter duringspontaneously occurring or induced arrhythmia. Measurements ofimpedances at points associated with tissue voltage of less than 0.5 mVor poor contact of the catheter to the cardiac tissue are discarded asare impedance measurements from mapping points that are too internal tothe anatomic shell or points with far-field low amplitude electrograms.The remaining points of measured impedance are evaluated to determine ifthere is an area of approximately 3.4±2 cm² or 2.4±1.8% of the atrialsurface area having plural low impedance (Z_(low)) points therein, whereZ_(low)≦Z_(min)+0.1(Z_(max)−Z_(min)) and where Z_(min) is the minimumimpedance measured and Z_(max) is the maximum impedance measured,wherein the existence of said area of said plural low impedance(Z_(low)) points therein indicates a focal arrhythmia caused by acircumscribed region of focal firing and wherein the absence of saidarea of said plural low impedance (Z_(low)) points therein indicates areentrant arrhythmia caused by a macroreentrant circuit.

The measurements may be displayed in the form of an iso-impedance map,which may appear on a video display to facilitate appropriate therapy,e.g., ablation.

DESCRIPTION OF THE DRAWING

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is an illustration including a block diagram of one exemplarysystem for measuring the impedance of cardiac tissue and providing aniso-impedance map thereof to enable the user of the system to determineif the patient's arrhythmia is the result of reentrant activation orfocal activation;

FIG. 2 is an exemplary prior art anterior-posterior view iso-activationmap of the right atrium of a patient exhibiting a focal arrhythmia; thesuccessful ablation site correlates with the region of earliestactivation as indicated by the red coloration.

FIG. 3 is an exemplary iso-impedance map of the patient of FIG. 2, withthe iso-impedance map of the right atrium being in a shallow leftanterior oblique projection slightly different from the view of FIG. 2,but produced using the method and system of this invention and showing afocal activation pattern as evidenced by the contiguous low impedancearea or CLIA.

FIG. 4A is an exemplary right anterior oblique iso-impedance map of theright atrium of another patient produced using the method and system ofthis invention and showing the patient undergoing a macroreentrant basedarrhythmia, i.e., isthmus dependent atrial flutter; as evidenced by theuniform impedance pattern with no CLIA.

FIG. 4B is another exemplary iso-impedance map of the right atrium ofthe patient of FIG. 4A, with this view being a left anterior obliquecaudal view showing a macroreentrant impedance map pattern, i.e.,uniform impedance with no CLIA.

FIG. 5 is an exemplary left anterior oblique view iso-impedance map ofthe right atrium in the patient of FIGS. 4A and 4B produced using themethod and system of this invention during an induced focal atrialtachycardia that occurred the same day and showing a contiguous lowimpedance area superior-laterally indicative of the focal-basedarrhythmia.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the various figures of the drawing wherein likereference characters refer to like parts, there is shown in FIG. 1 acardiac arrhythmia discrimination system 20 for determining themechanism of cardiac arrhythmia in a patient. The details of the system20 will be described later. Suffice it for now to state that inaccordance with one aspect of this invention the system 20 and itsmethod of use effects electro-anatomic mapping of cardiac arrhythmias tohelp differentiate the two most common mechanisms of these arrhythmias,i.e., focal firing based arrhythmias versus macroreentrant basedarrhythmias. To that end the system and method of this invention employsa catheter and associated circuitry to effect the measurement of cardiactissue impedance during an arrhythmia (either spontaneously occurring orinduced) and to produce an iso-impedance map of the cardiac tissue toenable the user of the system to determine if there is a contiguous areaof low impedance, e.g., an area of approximately 1.4-5.4 cm²representing 0.6-4.2% of the surface area of the chamber of interestwhich would reflect a group of some 400,000-600,000 cardiac myocytesbeing depolarized synchronously by after-depolarizations, that isdriving activation of the cardiac chamber. This contiguous low impedancearea, hereinafter referred to as a “CLIA”, results from the opening of amultitude of “stretch activated” channels in addition to the normalmembrane channels responsible for the normal cardiac action potential,in these focal arrhythmias. This is in contrast to the macroreentrantarrhythmias, where a single focal firing triggers off a reentrantcircuit where all the tissue is activated by the sequential opening ofthe standard membrane channels. These two mechanisms can bedifferentiated by the presence (focal firing) or absence (macroreentry)of a CLIA during the arrhythmia.

While the exemplary embodiment of the system and method of thisinvention entails the production of an iso-impedance map to indicate theparticular mechanism causing the arrhythmia, other systems and methodare contemplated by this invention to enable such discrimination. Thus,for example, the system 20 may be designed to merely produce aperceptible signal, e.g., visible or audible or both, indicating thepresence of a CLIA, instead of producing an iso-impedance map.

The successful identification of a CLIA requires that the cardiac tissueimpedance measurements are all due to accurate measurement of normalmyocardial cell impedance and are not artifactually low due to a numberof causes. Such artifactually low impedance readings include measurementof scar areas (exhibited by low voltage and low impedance) ormeasurements involving inadequate tissue contact by the measuringinstrument, e.g., the particular point measured may be too internal andtherefore the catheter's measuring tip may not have been in sufficientcontact with the endocardial tissue surface to get a true impedancemeasurement. Measurements of impedance values from fibrous tissue, suchas the fibrous valve rings, are also expected to exhibit low impedance.Also, as will be described later, the definition of low impedance isbased on the range of tissue impedances measured during the arrhythmiain the chamber of interest. Thus, artifactually high impedances need tobe discarded from the measurements made in the interest of accuracy.Examples of artifactually high impedances are those measured in thegreat veins and the coronary sinus, where the smaller contribution ofblood pool shunt impedance increases the measured impedance.

Various contact electrical mapping catheter systems are commerciallyavailable and can be used, with or without modification (as explainedbelow), for carrying out the methodology of this invention. One suchsystem is the NOGA® XP System that is available from Biosense Webster.That system utilizes a location detection technology, i.e., magneticsensor navigation, and is shown in FIG. 1. That system 20 includes adeflectable tip, multi-polar, mapping/ablation catheter 22 having a 2 mmor 4 mm tip, an associated magnetic field-generating location pad 24, anexternal location reference patch 26 and an electronic processing unit28. The catheter 22 includes a miniature passive magnetic field sensor.The location sensors determine the location and orientation of themapping catheter in six degrees of freedom (DOF) by sensing changes inthe magnetic field produced by the pad 24. The system's electronics arearranged to record and process activation time, tissue impedance, tissuevoltage and the position of the recording in three dimensional spacerelative to the electro-anatomic shell to provide the user with theelectrical parameters measured at each of the contact point in chamberof interest.

In accordance with one aspect of this invention the system includes avideo display or monitor 30. The monitor serves to provide a graphicvisual display in the form of an iso-impedance map of the chamber ofinterest and can also display the various measured parameters and thelocation data for the impedance and voltage measurements acquired by thecatheter 22. The iso-impedance map comprises a 3D color-codedreconstructed representation of the heart. The color-coding enablesquick visualization of the measured impedance of the mapped tissue, incorrelation with its anatomical location and is presented in real-time.The processing unit 28 includes software which takes the readingsacquired by the catheter 22 to provide the map. Some exemplary colorcoded maps are shown in FIGS. 3, 4A, 4B and 5.

In order to ensure that only readings which are indicative of truecardiac impedance are considered by the system, those impedance readingsrepresenting artifactually low or artifactually high impedancemeasurements are discarded and the remaining valid impedance points keptand analyzed. To that end, the processing unit may includesoftware/firmware to automate the data acquisition and processing (e.g.,the discarding of artifactually low and/or artifactually high impedancemeasurements) to produce the iso-impedance map. If the system doesn'tinclude such software/firmware the user of the system will decidewhether or not to use or discard measured impedance points. Thus,depending upon the construction of the system 20, either the user of thesystem or the software/firmware in the system evaluates the impedancemeasurements made by the system and discards those impedancemeasurements for any points associated with a “low” tissue voltage(e.g., less than 0.5 mV). So too, any points representative of afar-field electrogram configuration (i.e., low dV/dt) or prolongedduration or with highly fractionated electrograms are discarded. Tissueimpedance at too great a distance internal to the calculated radius ofthe arc of the electroanatomic shell (i.e., >1 mm internal to a 15degree spherical arc of curvature with an atrial shell diameter of 4 cmand a wall thickness of 1.5-2.0 mm as processed by the data processor)are also be discarded as likely representing low impedance due to poorcontact.

It is noted that pressure sensing ablation catheters are becomingavailable. Such catheters may be incorporated into the system of thisinvention to enable the user (or the system itself) to quantify thecontact made by the catheter to the cardiac tissue to thereby provide amore accurate way of identifying low impedance measurements resultingfrom poor contact. Other means may also be provided in the system toquantify or determine if good catheter contact is made to provide anaccurate measurement of tissue impedance at the contact point.

In any case, of those impedance measurement points that are kept, thesystem determines if each retained point exhibits low impedance(“Z_(low)”), wherein Z_(low)≦Z_(min)+0.1(Z_(max)−Z_(min)), where Z_(min)is the minimum impedance measured and Z_(max) is the maximum impedancemeasured. In addition the system determines the normal impedance(“Z_(normal)”) of the patient's cardiac tissue, whereinZ_(normal)≧Z_(min)+0.2(Z_(max)−Z_(min)). Those retained point are thenused by the system to produce an iso-impedance map, such as those shownin FIGS. 3 and 4A, 4B and 5, wherein the differing impedancemeasurements are color coded to the anatomy of the heart and the mapdisplayed on the video monitor 30.

In the exemplary maps shown in FIGS. 3 and 4A, 4B and 5 low impedancepoints of the cardiac tissue are shown by red, whereas normal impedancepoints are shown by purple, with intermediate impedance points betweenred and purple ranging from yellow to green to blue. In accordance withone aspect of this invention the resulting impedance map is evaluated bythe user to determine if there is an area of approximately 3.4±2 cm² or2.4±1.8% of the atrial surface area having plural Z_(low) pointstherein. Such a contiguous low impedance area (i.e., a CLIA) indicates afocal arrhythmia caused by a circumscribed region of focal firing,whereas the absence of a CLIA indicates reentrant arrhythmia caused by amacroreentrant circuit.

The maps of FIGS. 3, 4A, 4B and 5 represent respective impedance maps ofa portion of the heart of patients that have been produced using thesystem and method of this invention by measuring the impedance of thecardiac tissue during an SVT episode (either naturally occurring orinduced). Such maps can be used to characterize electrical activationpatterns of the chamber of interest and help to differentiate amacroreentrant activation pattern (uniform distribution of tissueimpedances that are >minimum impedance plus 20% of the differencebetween minimum and maximum tissue impedance) from a centripetalactivation pattern indicative of a focal site of origin. Areas ofconcentrated low impedance can also identify regions of tissue wherethere is rapid coherent conduction of electrical currents due tospecialized conducting bundles, e.g., Bachman's bundle, or theinfero-posterior intra-atrial pathway.

FIG. 3 is an iso-impedance map showing a patient (“Patient A”)undergoing an arrhythmia resulting from a focal firing mechanism. As canbe seen therein, a focal area of low impedance (CLIA), indicating afocal firing mechanism, is denoted by a contiguous low impedance area of1.4-5.8 cm² (the red-yellow area) located infero-septally adjacent tothe tricuspid annulus. If the areas of minimum impedance have a muchsmaller surface area, i.e., <1.0 sq. cm, they are not considered torepresent the critical mass of cells required for focal firing togenerate large enough local circuit currents to depolarize the rest ofthe cardiac chamber. The ablation points for the treatment of thispatient's condition are shown by the small circular red areas designated“ablation sites”. A comparable view iso-activation map (LAT map) ofPatient A produced in accordance with the prior art is shown in FIG. 2.

Turning now to FIG. 5 there are shown an iso-impedance map of anotherpatient (“Patient B”) undergoing an arrhythmia. This map reveals thatthe mechanism causing Patient B's arrhythmia is most compatible with afocal firing mechanism since the iso-impedance map shows a focal area oflow impedance (CLIA) supero-laterally in the right atrium. With thisknowledge appropriate therapy, e.g., radiofrequency ablation, wascarried out by directing radiofrequency energy from the catheter 22 tothe desired ablation points (not shown in FIG. 5) on Patient B's heart.After the ablation procedure had been accomplished the arrhythmia wasattempted to be re-induced in Patient B in order to check theeffectiveness of the ablation procedure. In this case, it was possibleto induce another arrhythmia in Patient B since Patient B was determinedto also have a macroreentrant based arrhythmia. In particular, FIGS. 4Aand 4B are iso-impedance map of Patient B taken shortly after theablation procedure for remediating Patient B's focal based arrhythmia.As can be seen in FIGS. 4A and 4B the iso-impedance maps are generallyuniform and there are is no focal area of low impedance (CLIA), therebyindicating that the mechanism causing Patient B's other arrhythmia wasmost compatible with macroreentry. With this knowledge additionalablation was performed on Patient B by directing radiofrequency energyfrom the catheter to the critical isthmus necessary to interrupt thereentrant circuit. After that ablation procedure programmed atrialstimulation was performed to determine if any arrhythmias were stillinducible in Patient B, and none were, thereby indicating that the twoablation procedures had been successful. The ablation points for PatientB are show by the red circles in the maps of FIGS. 4A, 4B and 5.

As should be appreciated by those skilled in the art, since Patient Bexhibited two different types of arrhythmias on the same day which arereflected in the different maps of FIGS. 4A, 4B and 5, it is unlikelythat Patient B's impedance map patterns are the result of somestructural abnormality, e.g., fibrosis, rather than the physiologiccharacteristics of the two arrhythmias as the heart's structure will nothave changed during the period of time between the two arrhythmias.

It should be noted that the theory on which the methodology of thisinvention is based is as follows: it is known that areas of focal firinginvolve cells that develop triggered automaticity and the usualmechanism results from the development of “after depolarizations”. Thisphysiologic mechanism is based on increased intracellular Ca⁺² fluxesthat cause an increased intracellular Ca⁺² concentration that persiststhroughout the cardiac cycle, and that generates delayed (with respectto the duration of the normal action potential) depolarizing inwardcurrents via the Na/Ca exchanger and via stretch activated cationicchannels. Experimental and computer modeling data suggests that togenerate enough local circuit current to overcome source-sink bufferingeffects of the syncytial nature of atrial tissue, and produceregenerative depolarizing wave fronts, that would activate a wholecardiac chamber, would take a volume of some 500,000-700,000 cardiacmuscle cells, that would all be firing off simultaneously but out ofsynch with the normal activation of the cardiac chamber, to generate thefocal arrhythmia. Because these additional channels are open, causingadditional trans-membrane currents to flow, the tissue impedancemeasured in this region should be appreciably lower than the rest of thecardiac chamber being activated by the normal channels that open togenerate the standard cardiac action potential. Whereas in an SVT due toa macroreentrant circuit activation, the cardiac chamber of interest,after an initiating out of sync, early depolarization from an errantgroup of cells with isolated non-repetitive focal firing, the recurrentactivation of the cardiac chamber of interest proceeds via thetransmission of local circuit currents and sequential activation of thechamber via the normal action potential dependent membrane channels.Thus, the pattern of tissue impedance is relatively uniform and normal,with no focal region of lower tissue impedance relative to the average,as clearly shown in the map of FIGS. 4A and 4B.

Indeed in one preliminary observation, in a group of patients with longRP supraventricular tachycardia, which can be due to either a focalfiring mechanism e.g., focal atrial tachycardia (AT) or a macroreentrantmechanism e.g., atrial flutter (AFI), these two different iso-impedancemap patterns are observed. For example, a focal AT exists if there is afocal contiguous area of 1.4-5.8 cm² of low impedance, i.e., ≦ theminimum chamber impedance plus 10% of the difference between minimum andmaximum chamber impedances, whereas a macroreentrant mechanism exhibitsno such focal area. The proof that these two different patterns of theiso-impedance maps do reflect these two different mechanisms is that theablation procedures deployed to terminate these arrhythmias, based onthe mechanisms defined by these different iso-impedance map patterns,are almost uniformly successful. Further still, in the case of the focallow impedance map pattern, not only does this identify the mechanism asbeing due to a site of focal firing, but it also localizes the region offocal firing, as the successful ablation site is usually contained inthe CLIA or within, 10-25 mm of the center of the CLIA.

Some preliminary impedance measurements in patients with SVT followingimplantable cardioverter defibrillator (ICD) insertion, which turns outto be due to either atrial flutter (a macroreentrant arrhythmia) or afocal atrial tachycardia (rapid focal firing of a group of atrial cells)have been made, in approximately 50 patients. These preliminaryimpedance maps seem to support the theory that the two arrhythmiamechanisms can be readily differentiated by the differences in theiso-impedance maps. The focal atrial tachycardias show a concentratedcontiguous area of relatively low impedance defined as ≦minimumimpedance plus 10% of the difference between the minimum and maximumimpedance. Furthermore, the area of low impedance seems to beapproximately 1.4-5.8 cm². The literature suggests that a focal atrialtachycardia that results from calcium action potentials due to “afterdepolarizations” would require 700,000 atrial cells synchronously firingto activate the entire atrium. Using data from the ultrastructureliterature, the surface area of a single atrial cell is approximately1.9×10⁻⁶ cm². Thus the surface area of a group of 700,000 atrial cellscausing a focal atrial tachycardia would be 1.33 cm², which is in therange of the measurements of the focal low impedance area found on theiso-impedance maps produced in accordance with this invention.

It is contemplated that in further refining the impedance mappingtechnique of this invention by using smaller surface area electrodecatheters, that reduce the contribution of the blood pool shuntimpedance which decreases the composite measured impedance, one can moreaccurately measure the actual tissue impedance, and may see a reductionin the contiguous low impedance surface area more commensurate with thatpredicted by atrial cell anatomic calculations, i.e., 1.33 cm².

The iso-impedance maps are displayed in an analogous fashion to voltageamplitude maps. This is accomplished, for example, by setting ahypothetical range of low impedance at less than or equal to the minimumtissue impedance measured +10% of the difference between the minimum andmaximum measured impedances with the color scheme of red; intermediateimpedances as between the minimum+10%×(maximum−minimum impedances) andthe minimum+20%×(maximum−minimum impedance) denoted by a colortransition from orange to yellow to green to blue, and normal impedanceas minimum impedance+20%×(maximum−minimum impedance) denoted by purple.Similarly, iso-voltage maps set minimum tissue voltage as <0.5 mVdenoted by red, normal voltage as >1.5 mV denoted by purple, andintermediate voltages as 0.5-1.5 mV denoted by the transition of colorsfrom orange to yellow to green to blue. It should be noted that theparticular colors chosen are arbitrary and other color schemes can beused for the various impedances or voltages displayed in the maps.

As will be appreciated by those skilled in the art, and as discussedabove, the existing technique of LAT mapping (such as shown in FIG. 2)can yield confusing results. For example, a focal atrial tachycardiaemanating from the intra-atrial septum or from the left atrium can crossthe atrial septum and activate the right atrium preferentially in aunidirectional fashion because of differences in refractoriness andconduction times through the various atrial conduction bundles, therebymimicking a macroreentrant circuit pattern when the actual arrhythmiamechanism is focal. In contradistinction, an iso-impedance map producedusing the methodology of this invention is not subject to suchconfusion, as a focal arrhythmia source generates focal contiguous areasof low impedance, i.e., a CLIA and not a uniform, normal impedance map.Thus, the methodology of subject invention enables one to differentiatean arrhythmia due to a macroreentrant circuit from one due to acircumscribed region of focal firing. In this regard, it is believedthat focal arrhythmias demonstrate a contiguous low impedance area(CLIA) due to opening of stretch activated channels and activation ofthe Na/Ca exchanger associated with after-depolarizations, from aconfined area that cause focal firing. This can be contrasted to themore uniform distribution of impedance believed to be likely observedwith macroreentrant arrhythmias due to the transmission of normal actionpotentials which result from the sequential opening of the standardionic membrane channels. In particular, focal atrial tachycardiaimpedance maps consistently show a typical pattern of an isolatedcontiguous low impedance area (CLIA) of 3.4±2.0 cm representing 2.4+1.8%of the total atrial surface area. Macroreentrant SVT impedance maps showno such CLIAs and have a more uniform impedance pattern. Thus, thecontiguous low impedance area in focal atrial tachycardia impedance mapscan help localize the region where RF ablation lesions should beconcentrated. It can also identify regions of coherent rapidlyconducting tissue, e.g., Bachman's bundle, or the infero-posteriorpathway insertion points. In addition, it is believed, that the subjectmethodology may be able to identify focal “mother” rotors throughout theleft atrium that may participate in the generation and maintenance ofatrial fibrillation and may be able to identify areas of CAFE (complexatrial fractionated electrograms) thought to reflect these “mother”rotors better than analog electrical recordings currently in use. Astill further advantage of the subject invention is that since currentusage of conventional contact mapping systems focuses on voltage andactivation mapping, differentiation of macroreentrant circuits fromfocal arrhythmias requires measurement of parameters at a large numberof sites throughout the chamber of interest, e.g., 75-150 points. Incontradistinction, the impedance mapping methodology of this inventionmay produce a viable map with fewer points, e.g., approximately 30-50,thereby reducing the time for the mapping procedure.

As should be appreciated from the foregoing by those skilled in the artthe system and methods of this invention enable one to readilydiscriminate a focal arrhythmia from reentrant arrhythmia by virtue ofthe existence or absence of a CLIA. To that end, the system is arrangedto measure and register plural valid impedance measurements to theanatomy of the cardiac chamber of interest, with either the user of thesystem or software/firmware in the system discarding artificiallyinaccurate impedance measurements to provide the valid impedancemeasurements. The system then analyzes the retained impedancemeasurements to determine if a CLIA exists and the results of theanalysis can be provided as an iso-impedance map and/or a perceptiblesignal to indicate the presence of a CLIA. In addition, the system andmethod of this invention can also be used to identify regions ofcoherent rapidly conducting tissue e.g., Bachman's bundle or theinferoposterior pathway insertion points, to identify focal “motherrotors” throughout the left atrium that may participate in thegeneration and maintenance of atrial fibrillation and to identify areasof CAFE (complex atrial/fractionated electrograms) that truly reflectthese mother rotors.

A further possible refinement of the subject invention may allow theidentification of actual reentrant pathway critical isthmuses and theirlocalization with high density impedance mapping. A high densitymultipolar mapping catheter, which is currently in development, shouldallow greater resolution in identifying regions of rapid coherentconduction (low impedance pathway) surrounded by areas of anisotropicconduction which forms regions of high impedance corridors due tofunctional conduction block. This could help direct ablation to such lowimpedance conductions pathways that form the critical isthmus of thereentrant pathway. In atypical right atrial flutters and most leftatrial flutters, LAT maps and entrainment mapping fail to reliablyidentify and localize these critical isthmuses.

Without further elaboration the foregoing will so fully illustrate myinvention that others may, by applying current or future knowledge,adopt the same for use under various conditions of service.

I claim:
 1. A cardiac arrhythmia discrimination system for determiningthe mechanism of cardiac arrhythmia in a patient, said cardiacarrhythmia discrimination system comprising a catheter and a processor,said system being arranged to measure the impedance of cardiac tissue ofthe patient at various selected points on the patient's heart during aspontaneously occurring or induced arrhythmia and providing thegeometric position of each of said points on the patient's heart, andwherein each of said measurements and positions are arranged to berecorded and analyzed by said processor to determine if a point on thepatient's heart exhibits low impedance (Z_(low)), whereinZ_(low)≦Z_(min)+0.1(Z_(max)−Z_(min)), where Z_(min) is the minimumimpedance measured and Z_(max) is the maximum impedance measured, saidprocessor being arranged to discard from its impedance measurements anypoints associated with tissue voltage of less than a minimum voltage orpoor contact of the catheter to the cardiac tissue and impedancemeasurements from mapping points that are too internal to the anatomicshell or points with far-field low amplitude electrograms, andevaluating the remaining points of measured impedance to determine ifthere is an area of a predetermined size or predetermined percentage ofthe atrial surface area having plural Z_(low) points therein.
 2. Thecardiac arrhythmia discrimination system of claim 1 wherein said minimumvoltage is 0.5 mV.
 3. The cardiac arrhythmia discrimination system ofclaim 1 wherein said area of predetermined size is 3.4±2 cm² and whereinsaid predetermined percentage of the atrial surface area is or 2.4±1.8%.4. The cardiac arrhythmia discrimination system of claim 2 wherein saidarea of predetermined size is 3.4±2 cm² and wherein said percentage ofthe atrial surface area is or 2.4±1.8%.
 5. The cardiac arrhythmiadiscrimination system of claim 1 wherein said system processor isarranged to determine the normal impedance (Z_(normal)) of the patient'scardiac tissue, wherein Z_(normal)≧Z_(min)+0.2(Z_(max)−Z_(min)).
 6. Thecardiac arrhythmia discrimination system of claim 1 wherein said pointsthat are too internal to the anatomic shell are >1 mm internal to a 15degree spherical arc of curvature with a radius of 1.5-2 cm inscribed bya local group of measured points on the heart chamber shell.
 7. Thecardiac arrhythmia discrimination system of claim 1 wherein said systemis arranged to produce a perceptible signal upon the detection of saidplural Z_(iow) points within said area.
 8. The cardiac arrhythmiadiscrimination system of claim 5 wherein said system comprises aprocessing unit and a video display coupled to said processing unit,said processing unit comprising said processor, said video display beingarranged to produce a iso-impedance map of the cardiac tissue, saidiso-impedance map being colored to represent differing impedancesmeasured by said catheter to enable a user of said cardiac arrhythmiadiscrimination system to visually analyze the color pattern of saidiso-impedance map to differentiate a focal arrhythmia caused by acircumscribed region of focal firing from reentrant arrhythmia caused bya macroreentrant circuit.
 9. The cardiac arrhythmia discriminationsystem of claim 1 wherein said system comprises a magnetic sensornavigation to provide the geometric position of each of said points onthe patient's heart.
 10. The cardiac arrhythmia discrimination system ofclaim 1 wherein said catheter is arranged to determine the amount ofpressure applied by it onto the cardiac tissue to enable said processorto determine if said catheter has made good contact with the cardiactissue.
 11. The cardiac arrhythmia discrimination system of claim 1wherein said catheter comprises a multipolar electrode catheter.
 12. Thecardiac arrhythmia discrimination system of claim 11 wherein saidcatheter is a radiofrequency ablation catheter.
 13. The cardiacarrhythmia discrimination system of claim 8 wherein Z_(low) isrepresented by the color red in said iso-impedance map and whereinZ_(normal) is represented by the color purple in said iso-impedance map,with intermediate impedances being represented by different colors. 14.A method of determining the mechanism of cardiac arrhythmia in a patientcomprising: a) measuring impedance of cardiac tissue of the patient atvarious selected points on the patient's heart using a catheter during aspontaneously occurring or induced arrhythmia; b) recording thegeometric position of each of said points on the patient's heart; c)discarding from said impedance measurements any points associated withtissue voltage of less than a minimum voltage or poor contact of thecatheter to the cardiac tissue and impedance measurements from mappingpoints that are too internal to the anatomic shell or points withfar-field low amplitude electrograms; and d) evaluating the remainingpoints of measured impedance to determine if there is an area of apredetermined size or predetermined percentage of the atrial surfacearea having plural low impedance (Z_(low)) points therein, whereZ_(low)≦Z_(min)+0.1(Z_(max)−Z_(min)) and where Z_(min) is the minimumimpedance measured and Z_(max) is the maximum impedance measured,wherein the existence of said area of said plural low impedance (Z_(low)points therein indicates a focal arrhythmia caused by a circumscribedregion of focal firing and wherein the absence of said area of saidplural low impedance (Z_(low) points therein indicates a reentrantarrhythmia caused by a macroreentrant circuit.
 15. The method of claim14 wherein said minimum voltage is 0.5 mV.
 16. The method of claim 14wherein said area of predetermined size is 3.4±2 cm² and wherein saidpredetermined percentage of the atrial surface area is or 2.4±1.8%. 17.The method of claim 15 wherein said area of predetermined size is 3.4±2cm² and wherein said percentage of the atrial surface area is or2.4±1.8%.
 18. The method of claim 14 wherein said points that are toointernal to the anatomic shell are >1 mm internal to a 15 degreespherical arc of curvature with a radius of 1.5-2 cm inscribed by alocal group of measured points on the heart chamber shell.
 19. Themethod of claim 17 wherein said points that are too internal to theanatomic shell are >1 mm internal to a 15 degree spherical arc ofcurvature with a radius of 1.5-2 cm inscribed by a local group ofmeasured points on the heart chamber shell.
 20. The method of claim 14wherein said method additionally comprises providing an iso-impedancemap of the cardiac tissue from said measurement, said iso-impedance mapbeing colored to represent differing impedances measured to enable aperson to visually analyze the color pattern of said iso-impedance mapto differentiate a focal arrhythmia caused by a circumscribed region offocal firing from reentrant arrhythmia caused by a macroreentrantcircuit.
 21. The method of claim 14 wherein said method is carried outduring an episode of supraventricular tachycardia in the patient. 22.The method of claim 14 additionally comprising ablating appropriatecardiac tissue resulting from the evaluation of the measurements. 23.The method of claim 20 additionally comprising ablating appropriatecardiac tissue resulting from the evaluation of the iso-impedance map.